Intellectual Merit: Carbon (C) in the form of CO₂ and methane, water vapor, and energy (originating from sunlight) are both absorbed and released by terrestrial ecosystems.  The balance between uptake and release of C, water, and energy from terrestrial ecosystems influences the Arctic system as a whole, and thus, how the Arctic affects weather in other parts of the world and global climate. Understanding how these balances will alter as climate changes is a major goal of the multiagency US Study of Environmental Arctic Change (SEARCH) and the NSF Arctic Observatory Network (AON) programs. Because cycles of C, water, and energy interact strongly with each other, they need to be studied in the same place and at the same time. This AON project was a collaboration between University of Alaska Fairbanks (UAF), the Marine Biological Laboratory, and the University of Michigan; the UAF portion focused on extending continuous, year-round measurements of C, water, and energy balance in terrestrial ecosystems in the Alaskan Arctic, which we have collected via eddy covariance since 2007. Long-term monitoring of hydrology was continued, and NDVI in the footprints of the eddy covariance towers were measured, at Imnavait Creek, Alaska.

At Imnavait Creek, we measured C uptake and release in three tundra ecosystems along a hillslope, at the top of a ridge (heath), mid-slope (tussock), and valley-bottom (wet sedge).  We found dramatically increased C loss over fall and winter between 2013 and 2016 at the wet sedge site, but the rate of loss has slowed since then, from 2017 and 2019. This loss was surprising, because, although the C content of soils at the wet sedge site is high, the soils are wet and would be expected to store C. However, permafrost temperature in a borehole near our site was the warmest north of the Brooks Range in the Thermal State of Permafrost network, and has increased steadily since 2006. Soil temperatures in late fall and early winter have continued to warm over this entire period, but it has been slightly cooler recently than it was in 2013-2016. Warm fall soil temperatures likely promote C loss, because soil microbial activity could continue at a high rate for a longer period of time. C loss correlated with the length of time in fall during which the soil is at 0 ^oC, even though air temperature is below freezing; this occurs because the latent heat of freezing water retards soil cooling. Together, these observations suggest that this area may be close to crossing a tipping point of permafrost thaw. In contrast, all three tundra ecosystems showed net C uptake during summer. Eddy covariance data showed that summer C uptake increased over time in the tussock and heath sites, but not in the wet sedge. Our biomass harvest in 2013 suggested that aboveground vascular plant biomass at our tussock site is nearly 4-fold higher than found at nearby sites at Imnavait Creek in the 1980s. Mobilization of soil C may have increased soil N availability, which could have increased plant growth in this N limited ecosystem.  However, gains in summer C uptake have not made up for winter C losses.

Broader Impacts: Data collected by this project contribute directly to the goals of SEARCH and AON, and have been used extensively by other researchers.  Our data are incorporated into observing networks such as Ameriflux and Phenocam, and have been used by several NASA missions. Undergraduate researchers and technical personnel have been trained, and results have been included in classes taught by the PIs and in public outreach, including for the national initiative Solve Climate by 2030, a climate action initiative to work towards state and local-level solutions to climate change. This project provided key professional development for the three female PIs, all of whom made significant career advances as a result.

Last Modified: 07/28/2020
Modified by: Marion S Bret-Harte